The invention relates to methods for removing polymeric coatings from optical fiber and apparatus for practicing such methods. In particular it relates to an advantageous method of removing such coatings by the application of heat within a low pressure environment.
Optical fibers are key components in modem telecommunications systems. Basically, an optical fiber is a thin strand of glass capable of transmitting optical signals containing a large amount of information over long distances with very low loss. In its simplest form, it is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. A polymeric coating surrounding the cladding protects the fiber from contamination and mechanical damage and maintains mechanical strength. Typical optical fibers are made of high purity silica glass with minor concentrations of dopants to control the index of refraction. Typical coatings are dual coatings of urethane acrylates. An inner (primary) coating having a relatively low in situ equilibrium modulus is applied directly to the glass, and an outer (secondary) coating having a relatively high modulus surrounds the primary coating.
While protective coatings are critical for most applications of optical fiber, short lengths of coating must be temporarily removed in the fabrication of optical fiber devices and during the assembly of fiber networks. Because the surface of the glass fiber is susceptible to damage from abrasion and contamination, the surface is coated with protective polymer immediately after the fiber is drawn. However the coating must be temporarily removed in the fabrication of important optical fiber devices such as fiber Bragg gratings, long period gratings, fused couplers and metalcoated regions. Moreover end portions of the coating need to be removed in fusing successive fiber segments to form a network. The fiber may be recoated after such operations.
A variety of approaches have been used to remove fiber polymeric coatings, but none have been completely satisfactory. One method is to mechanically scrape the coating off the glass using a blade and then to chemically clean the exposed glass. This approach inevitably creates surface flaws on the fiber, reducing the strength and the reliability of devices produced from it. A second method uses chemical solvents to soften or completely remove the coating. While this approach has been used in manufacturing, it is difficult to automate and involves the use of hazardous chemicals, typically in the form of concentrated acids at high temperatures. A third method uses heat to soften the coating and mechanical removal of the softened coating followed by ultrasonic cleaning with a solvent such as acetone or alcohol. Accordingly there is a need for an improved method of removing polymeric coatings from optical fiber.
In accordance with the invention, the polymeric coating is removed from a coated optical fiber by disposing the fiber within a low pressure environment and applying sufficient heat to volatilize at least a portion of the polymeric coating. The result is that the coating material bursts from the fiber, yielding a clean glass surface virtually free of surface flaws. In a preferred embodiment the heat is provided by a resistive filament heater within a vacuum chamber.